I talked to Ted at Engnbldr on Wednesday, and asked about something i saw he'd posted on YotaTech forum.
He says they flow-tested various headers against the stock 22R-RE exhaust manifold, and were puzzled cause there was No Difference. The stock Toyota manifold flows just as good as most headers, the difference comes from putting a 2" pipe back to a 2-1/4" muffler, with a 2-1/4" tailpipe after that.
The Toyota engineers designed a good free-flowing exhaust mainfold, but put a restrictive exhaust after it to pass emissions and cut down on the noise.
yes i agree with Ted, as far as "most" headers are concerned,